Blood pressure is normally measured by placing a blood pressure cuff around the arm of a patient over the brachial artery. The cuff typically includes an inflatable bladder placed in an outer casing. The bladder is inflated to compress the arm of the patient, thereby pinching off the flow of blood through the brachial artery. The pressure in the bladder is gradually reduced while listening for sounds caused by the flow of blood through the brachial artery and measuring the air pressure in the bladder. When blood flow is detected during systole, the air pressure in the bladder is recorded as the systolic blood pressure. Similarly, when blood flow is detected during diastole, the air pressure in the bladder is recorded as the diastolic blood pressure.
Although the most common device for measuring blood pressure using the above-described procedure is the familiar manually pumped cuff using a mercury manometer as the pressure measuring device, automated patient monitoring systems are also in common use.
One example of an instrument for automatically performing medical diagnostic tests is the ambulatory or bedside blood pressure monitor. These blood pressure monitors include a conventional blood pressure cuff connected through a tube to a monitoring instrument. The monitoring instrument includes an electric motor driving an air pump, a pressure traducer for measuring the air pressure in the cuff, and, if a separate transducer is not used, to also detect Korotkoff sounds or oscillometric pulses, which are generated by the flow of blood through the brachial artery. All of these components are generally controlled by a microprocessor. The monitoring instrument may also include a recording device, such as a magnetic tape recorder, or a digital display for providing a visual blood pressure indication.
In operation, the motor is energized to inflate the cuff while the pressure in the cuff is monitored by the pressure transducer. When the cuff pressure reaches a predetermined value, the processor periodically actuates an air valve to incrementally bleed air from the cuff thereby reducing the cuff pressure. At each cuff pressure value, the transducer measures the cuff pressure and detects Korotkoff sounds or oscillometric pulses. The processor, using a rather complex algorithm, then determines the blood pressure from a table of cuff pressures and data indicating whether Korotkoff sounds or oscillometric pulses are detected at each cuff pressure. The blood pressure is then either recorded or displayed.
Although automated blood pressure monitors of the type described above allow the blood pressure of a patient to be easily measured without the need for trained personnel, they have disadvantages. Most significantly perhaps, they utilize numerous tubes and connections, which results in the leakage of air from the system. This inadvertent leakage can adversely affect the accuracy of blood pressure measurements because the air pressure in the cuff will be changing while a measurement is being taken. Furthermore, the rate of leakage is not uniform, so the leakage cannot be compensated for by the monitor.
In addition, conventional automated blood pressure monitoring systems tend to be bulky and somewhat expensive, and to have several wear points. A need therefore exists for a blood pressure monitoring device that will lose a minimum of air through inadvertent leakage, and that will be compact and resist wear.